Microsoft Investigating MHTML Vulnerability in IE

Microsoft released Security Advisory 2501696 in response a scripting vulnerability in Internet Explorer that affects all versions of Windows.

The security hazard is found in the MIME Encapsulation of Aggregate HTML (MHTML) protocol handler. The attach mechanism is similar to a server-side cross-site-scripting (XSS) exploit, in which a malicious script could run on a user's computer after clicking on a link.

While this vulnerability could be exploited by hackers, the chances of an attack are slim, according to some software security analysts.

"At first glance today's advisory looks grim because it affects every supported Windows platform," wrote Andrew Storms, director of information and technology at software security firm nCircle, in a released statement. "However, even though the proof of concept code is public, carrying out an attack using this complicated cross site scripting-like bug will not be easy."

While there currently is no patch to fix the vulnerability, Microsoft suggested a workaround. This mitigation approach disables MHTML handler scripting by setting all corresponding keys in the Windows registry. Microsoft issued a "Fix it" in a KnowledgeBase article to automate the workaround.

Wolfgang Kandek, CTO of Qualys, offered another option to avoid the vulnerability: use an alternative browser.

"While the vulnerability is located in a Windows component, Internet Explorer is the only known attacker vector," Kandek wrote in a blog posting. "Firefox and Chrome are not affected in their default configuration, as they do not support MHTML without the installation of specific add-on modules."

The hole was first brought to the attention by individuals on the WooYun Web site. This is the same site that divulged information about the vulnerability in the CSS handler of Internet Explorer in December.

Coupled with a handful of security concerns earlier this month, Microsoft has been busy with multiple vulnerabilities as of late. "2011 is not off to an auspicious start for Microsoft's security staff," wrote Storms. "In early January Jonathan Ness posted an explanation of five public security bugs Microsoft was tracking to the SRD blog. Today, just two short weeks later, we have another one to add to the list."